Several techniques have been developed in recent years for the automatic detection and localization of microseismic events associated with hydraulic fracturing treatments in unconventional resource plays, providing large advancements in the reliability of microseismic analysis. However, these modern techniques are still adversely affected by the presence of multipath arrivals of microseismic energy. In many unconventional shale plays, the resource bearing layer is located immediately above and/or below a more dense, high-velocity layer that gives rise to complex wave propagation where energy from a single microseismic event will reach an array of geophones near target depth through multiple indirect propagation paths such as reflections and head waves. If not correctly identified and analyzed, these indirectly arriving wavefronts can cause false positive detection of additional events believed to originate across the layer interfaces that generate them (see B. Fuller et al., Seismic wave phenomena and implications for accuracy of microseismic results, Recorder, 2010). Additionally, since head waves are radiated by critically refracted waves that travel at a higher velocity than the direct path for events occurring within the target layer, they can arrive either before or after the direct-path wavefront, depending on a source's distance from the monitoring array. This property of head waves can cause further complications for standard analysis techniques, which typically look to only the first arrival associated with any particular microseismic event.
Previous discussions of these multipath complications have typically suggested designing monitoring arrays to avoid head waves and other indirect-path arrivals (see B. Fuller et al., Seismic wave phenomena and implications for accuracy of microseismic results, Recorder, 2010; U. Zimmer, Localization of microseismic events using headwaves and direct waves, SEG Expanded Abstracts, 2010). More recent discussions of head waves in microseismic analysis have begun to address combining earlier arriving head waves with later direct-path arrivals during localization for improved accuracy (U. Zimmer, Localization of microseismic events using headwaves and direct waves, SEG Expanded Abstracts, 2010; Microseismic design studies, Geophysics, 2011), but have not addressed the combination of these two arrival types in general and have assessed the benefit of their combination only through travel-time-residual analysis, which obscures the underlying localization geometry and fails to provide an intuitive understanding of the associated effects on accuracy.
Relatedly, many important unconventional shale assets are hosted within regions where geological formations near the target layer create multiple paths through which the energy from a single microseismic event can reach an array of geophones positioned near target depth. These indirect arrivals, which include reflections and head waves, have traditionally been viewed as a nuisance in microseismic analysis, potentially causing additional false-positive event detections and erroneous localizations into the layers which generate them. In particular, the effects and potential use of head waves in microseismic analysis have been studied recently (see B. Fuller et al., Seismic wave phenomena and implications for accuracy of microseismic results, Recorder, 2010; U. Zimmer, Localization of microseismic events using headwaves and direct waves, SEG Expanded Abstracts, 2010; Challenges and solutions in locating and interpreting microseismic events from surveys in the Horn River Basin, CSPG CSEG CWLS Convention, 2011; Microseismic design studies, Geophysics, 2011); however, these studies have focused on presurvey design to avoid observing head waves and have discussed the use of direct-path and head-wave arrivals in combination for event localization only in limited cases.
For example, typical microseismic processing techniques aim to analyze only the first or direct-path arrivals of microseismic event energy during localization. This approach affords limited event depth estimation accuracy in surveys where the source-to-receiver-array distances are large, due to typically small relative array apertures, and is adversely affected by the presence of multiple, indirect-path arrivals. Such arrivals, including mode conversions, reflections and head waves, are generated whenever geological formations of contrasting propagation velocity exist near the target layer. In particular, the presence of high-velocity formations near the target layer will give rise to head waves—plane waves that are radiated back into the lower-velocity target formation by critically refracted wavefronts traveling along the interface between the two layers at the higher speed. Such geological arrangements are typical in many unconventional shale plays, including the Montney in Alberta, Canada, which is bounded below by the high-velocity Belloy formation. Improper identification and analysis of these head waves can cause gross localization errors of microseismic events across the head-wave-generating interface and into the high-velocity layer.